Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation

[Peter Zijlstra]

On Tue, Apr 16, 2019 at 05:49:37PM +0200, Peter Zijlstra wrote:
> See, if you first write that function in the form:
> 
> 	long new;
> 
> 	do {
> 		new = count | RWSEM_WRITER_LOCKED;
> 
> 		if (count & RWSEM_LOCK_MASK)
> 			return false;
> 
> 		if (list_is_singular(&sem->wait_list))
> 			new &= ~RWSEM_FLAG_WAITERS;
> 
> 	} while (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
> 
> 	rwsem_set_owner(sem);
> 	return true;
> 
> And then add the HANDOFF bits like:
> 
> 	long new;
> 
> 	do {
> +		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
> 
> +		new = (count | RWSEM_WRITER_LOCKED) & ~RWSEM_FLAG_HANDOFF;
> 
> 		if (count & RWSEM_LOCK_MASK) {
> +			if (has_handoff && wstate != WRITER_HANDOFF)
> +				return false;
> 			new |= RWSEM_FLAG_HANDOFF;
> 		}
> 
> +		if (has_handoff && wstate == WRITER_NOT_FIRST)
> +			return false;
> 
> 		if (list_is_singular(&sem->wait_list))
> 			new &= ~RWSEM_FLAG_WAITERS;
> 
> 	} while (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));

obviously that should be !

> 
> 	rwsem_set_owner(sem);
> 	return true;
> 
> it almost looks like sensible code.